As the rear swing arm for a motor-bike, a tapered pipe having a longitudinal rectangular sectional surface has been used. As one example of this manufacturing method, there are methods disclosed in Japanese Patent Laid-open Nos. Hei 2-286488 and 2-286489.
These methods are operated such that at first a round raw pipe is formed. The raw pipe is then formed into a tapered pipe in a rotary swagging step where some dies are rotated around the raw pipe to crush it and then lastly form the pipe into a rectangular sectional shape by a bulging step or a compression forming step.
In view of the foregoing, the aforesaid prior art examples require a total number of two steps in the rotary swagging step and the subsequent bulging step or compression forming step in order to obtain the rectangular pipe from the round raw pipe. Accordingly, the number of steps is increased.
In addition, due to application of the rotary swagging step, a degree of freedom of the shape is reduced by the facts that the taper has an axial symmetrical formation and an arcuate (an angle R) shape at the corner part of the rectangular pipe which results in certain restrictions.
In addition to this fact, although each of the wall parts within a longitudinal sectional surface shows a certain variation in wall thickness, each of the walls within the cross-sectional surface, i.e. a sectional surface crossing at a right angle with a longitudinal direction has a uniform wall thickness. Accordingly, it is not possible to optionally set a wall thickness at each of the wall parts within the cross-sectional surface and to efficiently assure a section modulus.
In addition, since an application of load in the rear swing arm of a motor-bike is different in each of the upper, lower, rightward or leftward directions, respectively, it is desirable to form the swing arm with irregular wall thicknessed in which each of the upper, lower, rightward or leftward directions in the cross-sectional surface of the pipe member has a different wall thickness, respectively. However, such an irregular thickness wall formation is impossible in the aforesaid prior art forming method. In addition, it is desired to have a large ground clearance or a long suspension stroke with the rear swing arm.